Pelton noted that part of the reason big guys shoot worse is that
there are so few of them. He
writes:

"As was noted at Sloan, part of the reason big men tend to be
worse as shooters is that the smaller pool of extremely tall
players and limited need for them to shoot well allows teams to
tolerate a lack of skill they would not accept at other
positions."

"Besides height, the physical trait that marks most NBA
basketball players is that they have unusually long arms and, in
many cases, enormous hands. This is tremendously useful on a
basketball court in general, but at the free throw line it's an
active impediment. It's very hard for a long-armed player to have
a compact, quick shot release -- instead, for many, they can
scarcely get out of their arms' way and end up with a slower,
looping release."